net: ti: cpmac: Use the correct function to free some resources.

In 'cpmac_open', 'dma_alloc_coherent' has been used to allocate some
resources, so we need to free them using 'dma_free_coherent' instead
of 'kfree'.

Also, we don't need to free these resources if the allocation has failed.
So I have slighly modified the goto label in this case.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>